A Bronze Matryoshka: The Discrete Intermetalloid Cluster [Sn@Cu12@Sn20]12– in the Ternary Phases A12Cu12Sn21 (A = Na, K)

Abstract

The synthesis and crystal structure of the first ternary A–Cu–Sn intermetallic phases for the heavier alkali metals A = Na to Cs is reported. The title compounds A₁₂Cu₁₂Sn₂₁ show discrete 33-atom intermetalloid Cu–Sn clusters {[email protected]₁₂@Sn₂₀}, which are composed of {Sn₂₀} pentagonal dodecahedra surrounding {Cu₁₂} icosahedra with single Sn atoms at the center. Na₁₂Cu₁₂Sn₂₁ and K₁₂Cu₁₂Sn₂₁ were characterized by single-crystal XRD studies, and the successful synthesis of analogous A–Cu–Sn compounds with A = Rb and Cs is deduced from powder XRD data. The isotypic A₁₂Cu₁₂Sn₂₁ phases crystallize in the cubic space group Pn3̅m (No. 224), with the Cu–Sn clusters adopting a face centered cubic arrangement. A formal charge of 12– can be assigned to the {[email protected]₁₂@Sn₂₀} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [[email protected]₁₂@Sn₂₀]^12– clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na₁₂Cu₁₂Sn₂₁ and K₁₂Cu₁₂Sn₂₁, DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [[email protected]₁₂@Sn₂₀]^12– cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [[email protected]₁₂@Sn₂₀]^12– cluster MOs can be classified with labels indicating the numbers of radial and angular nodes, in the style of spherical shell models of cluster bonding.